Chopping is a common technique for reducing offset voltage, offset drift, and 1/f noise in circuits. This method translates DC and flicker noise to higher frequencies, an artifact of which is chopper noise or ripple. Switched capacitor filters are well suited for ripple suppression due to their high selectivity and rejection. The use of switched capacitor filters to reduce ripple is well known in the art.
The offset voltage of a differential amplifier is the voltage that needs to be added or subtracted from one of the input voltages so that, when the input voltages are equal, the output of the amplifier is precisely zero. The offset voltage gets amplified by the amplifier and downstream amplifiers.
One prior art technique for reducing ripple in a chopped operational amplifier is described in U.S. Pat. No. 7,292,095 to Burt, incorporated herein by reference, where the representative figures from that patent are reproduced as FIGS. 1A and 1B.
In the Burt circuit, a conventional input chopper 9 is connected between a differential input voltage Vin and a trans-conductance amplifier 2 to reverse the input polarity at a 50% duty cycle, and a synchronized output chopper 10 is connected to the differential output of the amplifier 2 to correct the signal path polarity. The equivalent input offset voltage of the amplifier 2 contributes equally during both phases, but appears in alternating polarities at the output of the output chopper 10. Therefore, the offset going into subsequent stages is ideally nullified, but the ripple caused by chopping is also added to the output signal. A switched capacitor notch filter 15, operating at precisely 90 degrees out of phase with the choppers (see the phase 3 and 4 signals), filters out the ripple at the chopper frequency. The remainder of the Burt circuit is related to frequency compensation and is not relevant to the present invention, which is a filter for removing chopper ripple.
Ripple at the output of the output chopper 10 due to offset in the amplifier 2 will be a square wave current that is synchronous with phase 1 and 2 (FIG. 1B). When integrated onto a capacitor, this square wave current will result in a triangle wave voltage. At the mid-point in time between the phase 1 and phase 2 chopping transitions, this differential voltage will be at the mid-point between the peaks and valleys of the triangle wave, ideally at the same exact voltage in the rising direction as well as the falling direction. An integrate-and-transfer function from one such mid-point to the next will result in a consistent output, thus greatly suppressing the ripple. These mid-points occur twice in each chopper cycle, so the filter ping-pongs between the capacitor C5 and capacitor C6 paths, each alternately integrating while the other is holding.
In the above-described Burt circuit, if the switched capacitor notch filter 15 is not operated at exactly 90 degrees out of phase with the choppers, there will still be some residual ripple in the generated signal. Furthermore, the notch filter 15 utilizes two ping-ponged signal paths, which adds circuit complexity and area.
What is needed is an improved notch filter design of the type that reduces or eliminates chopper ripple, does not require the generation of a control signal that is precisely 90 degrees out of phase with the chopper, and offers a reduction in circuit complexity and device area.